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A Case Study in Rock Harmony

I covered the 1, 4, 5 chord progression in detail a couple of weeks ago in the “Small Victory” thread. I have been revisiting a few songs that I have not played in over thirty years that are not based on the 1, 4, 5 chord progression. I would like to share an analysis of one of the most common non-1, 4, 5 progressions that was used in seventies rock; namely, the 1, b7, 4 progression (b7 means the seventh chord in the key is lowed by a half step or one fret in guitar terms). This progression was used in southern rock tunes such as “Sweet Home Alabama” by Lynyrd Skynyrd and “Can’t You See” by the Marshall Tucker Band. It was also used in “Thank You” by Led Zeppelin and “Mr. Fantasy” by Traffic.

The chord progression to “Sweet Home Alabama” and “Can’t You See” is D (1 chord), C, (b7 chord) and G (4 chord). The main difference between these songs is that “Can’t You See” releases back to the tonic (1 or root) chord D whereas “Sweet Home Alabama” hangs on the subdominant (4) chord G. Many rock guitarists believe that both songs are in the key of D major; however, neither song is in the key of D major, as the chord C major does not appear in the key of D major. The notes in D major are D, E, F#, G, A, B, and C#.

With the above said, how do we determine the key for these songs? The key to understanding the musical key in which these songs are written is the b7 chord C. If we rearrange the chords D, C, and G, we realize that they are the 1, 4, and 5 chords in the key of G major. The notes in the key of G major are G, A, B, C, D, E, and F#. As one can clearly see, the musical keys G major and D major only differ by one note. The chords in the key of G major are G, Am, Bm, C, D, Em, and F#dim(F#m is often used as a substitute). The chords in the key of D major are D, Em, F#m , G, A, Bm, and C#dim (C#m is often used as a substitute).

Now, anyone who has seen the Marshall Tucker Band play “Can’t You See” is probably questioning my sanity.

The guitarists clearly use what looks like the B minor pentatonic pattern in the solos. As I covered in the “Small Victory” thread, B natural minor is the relative minor key of D major; therefore, B natural minor contains the same notes as D major.

D Major (Ionian mode)

B Natural Minor (Aeolian mode or simply Bm)

The reason why what appears to be a key assignment mistake works is because the pentatonic scale is missing two notes (one of which is the note C#). The missing notes are numbered 4 and 7 in the major pentatonic and 2 and 6 in the relative minor pentatonic.

D Major Pentatonic

B Minor Pentatonic

How can a song start on chord other than the root chord for the key? Making a note other than root note in a musical key a song’s tonal center is what is known as a mode.

When compared to the major scale (Ionian mode),

Starting on the 2nd scale degree and playing to the 2nd scale degree one octave up or down is Dorian mode.

Starting on the 3rd scale degree and playing to the 3rd scale degree one octave up or down is Phrygian mode

Starting on the 4th scale degree and playing to the 4th scale degree one octave up or down is Lydian mode

Starting on the 5th scale degree and playing to the 5th scale degree one octave up or down is Mixolydian mode

Starting on the 6th scale degree and playing to the 6th scale degree one octave up or down is Aeolian mode

Starting on the 7th scale degree and playing to the 7th scale degree one octave up or down is Locrian mode

If we examine the notes in the key of G, we see that the fifth note is D; therefore, “Sweet Home Alabama” and “Can’t You See,” are in D Mixolydian mode, which contains the same notes and chords as the key of G. The solos in “Sweet Home Alabama” are in the key of G major with a little E minor tonality thrown in for spice.

G Major (Ionian mode)

E Natural Minor (Aeolian mode)

If we examine the pattern for B Phrygian mode, it becomes clear why we can solo in B minor pentatonic over the progression to “Can’t You See.” B Phrygian contains the same notes as G major, and all of the notes in B minor pentatonic are in B Phrygian (B minor pentatonic is what is known as a proper subset of B Phrygian in set theory terms). B Phrygian has the same note interval relationship to D Mixolydian as the natural minor (Aeolian mode) has to the major (Ionian mode); therefore, we can play D Mixolydian in the B Phrygian pattern just by starting on the same position in the pattern that we do when playing D major in the B natural minor/D major pattern. Playing the extra notes in B Phrygian allows us to pick up the 4th and b7th notes in D Mixolydian, which are the root notes for the chords G and C.

B Phrygian

D Mixolydian

Finally, is it possible to solo to a D, C, and G progression in D minor pentatonic? Well, it is possible, but there is a catch. D minor pentatonic contains a minor 3rd instead of a major 3rd (F instead of F#). This note has to be handled with care because it is a half of a step down from the parallel note in D Mixolydian. D minor pentatonic is the relative minor pentatonic of the key of F major. The notes in F major are F, G, A, Bb, C, D, and E, which makes the notes in D minor D, E, F, G, A, Bb, and C. As the minor pentatonic scale is missing the 2nd and 6th notes from natural minor scale, D minor pentatonic contains the notes D, F, G, A, and C, which gives us the rote notes for the cords D, C, and G, but the F note can be problematic.

In closing, if one takes anything away from this discussion, it should be that a 1, b7, 4 chord progression is a Mixolydian mode chord progression, which means that the musical key is seven half steps down or five half steps up from the root note of the root chord in the progression. Finding the musical key of a song is critical to soloing in key.